Method for controlling a vacuum valve of a vacuum diecasting device and a vacuum diecasting device

ABSTRACT

A vacuum die casting apparatus includes a casting cavity ( 10 ), which is evacuatable via a vacuum valve ( 26 ). A liquid casting material is pressable into the casting cavity by a piston ( 14 ) actuated by an actuator ( 17 ). A filling level sensor ( 20 ) detects a predetermined filling level of the casting material in the casting cavity. A control device ( 34 ) is connected to the filling level sensor for controlling the vacuum valve, and a position sensor ( 32 ) is connected to the control device ( 34 ) for detecting movement of the piston ( 14 ). The control device generates a closing signal for the vacuum valve ( 26 ) when the piston, after reaching the position (s 1 ) at which the filling level sensor ( 20 ) indicates a predetermined filling level of the casting cavity ( 10 ) with casting material has been reached, is displaced in a predetermined manner.

CROSS-REFERENCE

This application is the national stage filing of International patentapplication no. PCT/EP02/10234 filed Sep. 12, 2002, which claimedpriority to German patent application no. 101 44 945.3 filed Sep. 12,2001.

TECHNICAL FIELD

The invention concerns a method for controlling a vacuum valve of avacuum die casting apparatus and a vacuum die casting apparatus.

THE RELATED ART

A vacuum die casting apparatus is known from DE 200 16 166 U1, in whichthe filling level of liquid metal injected into the casting cavity isdetected by a metal contact sensor; upon coming into contact with theliquid metal, the sensor outputs a signal. A closing piston of thevacuum valve, which is driven by a linear motor, is moved in response tothe signal so that the vacuum valve is entirely closed when the metalreaches the vacuum valve. In a further teaching, the known vacuum diecasting apparatus utilizes a meandering vacuum and venting passage, inwhich a plurality of metal contact sensors are disposed and areconnected to a computer; the computer controls the linear motor of theclosing piston in accordance with the signals from the sensors.

If there is only one metal contact sensor, it is not possible tocompensate for fluctuations in the speed at which the molten metal isinjected during the manufacture of the castings; therefore, if thevacuum valve is not timely closed at high filling speeds, the evacuationwill be prematurely ended, thereby possibly detrimentally influencingthe quality of the casting. The use of a plurality of metal contactsensors is comparatively complicated and expensive.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method and an apparatus thatcan manufacture castings of improved quality.

According to one method of the invention, the position of a piston,which presses or injects liquid casting material into a mold cavity, isdetected, and the vacuum valve is closed in dependence on the pistonposition; as a result, it is possible for the vacuum valve to bereliably closed just before the casting material, preferably liquidmetal, reaches the vacuum valve. In that way, the casting mold or thecasting cavity can be connected to a vacuum source for as long aspossible; thus, polluting gases, i.e., gases originating from lubricantsand separating substances and the like, are suctioned away and a castingis produced that is substantially free of pores or inclusions.

A vacuum die casting apparatus according to the invention includes acasting cavity that is evacuatable via a vacuum valve. A liquid castingmaterial is pressable into the casting cavity by means of a pistonactuated by an actuator. A filling level sensor detects a predeterminedfilling level of the casting material in the casting cavity. A positionsensor detects the movement of the piston. A control device is connectedto the filling level sensor and the position sensor. The control devicegenerates a closing signal for the vacuum valve when the piston, afterreaching a position at which the filling level sensor indicates apredetermined filling level of the casting cavity with casting material,is displaced in a predetermined manner.

In another aspect of the invention, the vacuum valve advantageouslyoperates with a time delay that is as short as possible so that theclosing signal can be generated as late as possible; as a result, theclosing time is as late as possible, and is substantially independent offluctuations that occur during the operation of the casting apparatus.

In accordance with another aspect of the invention, the time point atwhich the closing signal must be generated can be calculated from thepiston speed almost in real-time.

Advantageous characteristic curves may be stored in the control device,and the operating condition of the die casting apparatus can bemonitored.

Further, operating fluctuations of the casting apparatus are taken intoaccount when generating the closing signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a vacuum die casting apparatus.

FIGS. 2 and 3 show curves to illustrate an operational mode of theapparatus according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, a vacuum die casting apparatus includes at least twomain bodies 2 and 4 that are movable towards and away from each other bymeans of a drive device (not shown) in a known manner. An associatedmold portion 6 and 8 is respectively fixed to each main body. Mutuallyfacing surfaces of the mold portions 6 and 8 are configured such that acasting cavity 10 is formed when the apparatus is closed or when themain bodies are moved towards each other.

A cylinder 12 opens towards the casting cavity 10 and contains a movablepiston 14. The cylinder 12 serves to introduce casting material,preferably liquid metal, into the casting cavity 10; the piston 14 isconnected via a shaft 16 to an actuator 17 for driving the piston 14. Afilling conduit 18 passes through the wall of the cylinder 12 forintroducing the liquid casting material.

A filling level sensor 20 is disposed adjoining the casting cavity 10,which filling level sensor 20 generates a signal upon contacting thecasting material 22 that rises in the casting cavity 10 when the castingcavity is filled with liquid casting material. A vacuum opening 24adjoins the upper end region of the casting cavity 10 and the vacuumopening 24 is connected via a vacuum valve 26 to a vacuum pump 28. Thevacuum valve 26 is configured such that, in response to a closingsignal, the valve member of the vacuum valve moves with the shortestpossible delay into the valve closed position. Actuation of the valvemember is effected, for example, by means of an actuator, e.g., asolenoid 30. The actuation can also be suitably effected hydraulicallyor by other means.

The stroke position of the piston 14 or the shaft 16 is detected by amotion sensor 32. The motion sensor 32 can be, for example, a linearsensor. In the alternative, if the linear movement of the piston isconverted into a rotational movement by a suitable mechanism, the motionsensor 32 may be an incremental rotational motion sensor.

The motion sensor 32, the filling level sensor 20 and the solenoid ofthe vacuum valve 26 are connected to a control device 34 that includes amicroprocessor with associated storage devices, a display unit, forexample in the form of a display, and operating elements.

The structure and function of the described components are known. Theco-operation of the components is described in the following:

It will be assumed that the piston 14 has been moved towards the rightin FIG. 1 beyond the outlet port of the filling conduit 18. At thatposition, liquid metal can be introduced into the evacuated castingcavity 10 via the conduit 18. For example, the space within the cylinder12 to the left of the piston 14 is filled to between 20% and 60% withliquid casting material, depending on the volume relationships thereof.The piston 14 is then moved towards the left beyond the outlet port ofthe filling conduit 18. The vacuum valve 26 is open at this time. Thevacuum pump 28 is operating so that the casting cavity 10 is subjectedto a vacuum. The casting material 22 is pressed into the casting cavity10 and eventually reaches the filling level sensor 20.

As soon as the casting material 22 reaches the filling level sensor 20,the filling level sensor 20 sends a trigger signal to the control device34; at the time the trigger signal is sent, the position of the piston14 is detected by the motion sensor 32 and the detected piston positionis stored in the control device 34. Advantageously, the actuator 17,e.g. a hydraulic actuator, is switched to a higher speed by the triggersignal so that the casting material is injected at a higher speed andunder a higher pressure into the remainder of the mold cavity.

FIG. 2 clarifies these relationships. The speed of the piston 14 isillustrated as being dependent upon the distance s that the piston 14has been displaced. As can be clearly seen, the piston movement beginsat a low speed until the position s₁, at which position the castingmaterial 22 reaches the filling level sensor 20. The associated positions₁ is stored in the control device 34. At the same time, the force or,if provided, the speed, with which the actuator 17 drives the piston 14,is switched to a high value. It will be assumed that the position s₂ ofthe piston 14 corresponds to the position at which the casting materialreaches the vacuum opening 24 of the vacuum valve 26; in other words, itis the piston position at which the vacuum valve 26 must be closed inorder to avoid being damaged by the casting material. It will further beassumed that the piston moves by the distance Δs within thepredetermined time delay period Δt that is required to close the vacuumvalve 26 in response to the generation of a closing signal. Therefore,as can be readily seen from FIG. 2, the closing signal for the vacuumvalve 26 must be generated at the time when the piston is disposed atthe position s₃. Because the position s₁ of the piston is stored in thecontrol device 34, the distance s₃-s₂ or the position s₃ of the piston14 can be accurately detected by the motion sensor 32, and the closingsignal for the vacuum valve 26 can be generated at that time.

FIG. 3 provides a characteristic curve indicating the change of thepiston position over time. FIG. 3 additionally shows the time delay Δtof the vacuum valve 26.

It will be appreciated that the characteristic curve of FIG. 2 can begenerated by differentiation of the characteristic curve of FIG. 3.Depending on the particular design configuration of the motion sensor32, the characteristic curve of FIG. 2 or FIG. 3 can be recordeddirectly. Thus, the characteristic curve of FIG. 3 can be generated byintegration of the characteristic curve of FIG. 2 or the characteristiccurve of FIG. 2 can be generated by differentiation of thecharacteristic curve of FIG. 3.

Depending upon the construction of the vacuum valve and the arrangementthereof in the casting cavity, it is possible to completely fill thecasting cavity when the piston 14 reaches the position S₂ (castingmaterial reaches the vacuum opening 24); or, as illustrated in theFigures, the casting cavity can be only substantially filled, so thatthe piston is still displaced a short additional distance. It will beappreciated that the piston can also be controlled in such a way thatthe piston is braked at the position s₂.

It is advantageous for the characteristic curve of FIG. 2 to be storedas a target or reference characteristic curve that corresponds to afunctionally acceptable operational condition of the casting apparatus.That reference characteristic curve can then serve to determine thepiston position s₃ for generating the closing signal for the vacuumvalve.

The reference characteristic curve can be continuously displayedtogether with an actual characteristic curve, for example on a displayscreen, so that operating changes in the die casting apparatus aredirectly visible and any faults can be recognised at an early stage. Inaddition, in the series production of castings, the characteristiccurve, which curve is used to ascertain the position s₃, can becontinuously updated. For example, the actual characteristic curve of animmediately preceding casting operation may be used as thecharacteristic curve for ascertaining s₃, or a characteristic curve maybe used that is derived from a plurality of preceding castingoperations.

Overall, the invention provides that the casting cavity is subjected tothe effect of a vacuum as long as possible during the casting operationso that high quality vacuum die castings are produced. The eliminationof pores in the die castings is further improved due to the fact thatthe final phase of the casting procedure is performed under a highcasting pressure provided by the higher piston force. Thus, if any poresare not suctioned away by the vacuum and remain in the die casting, dueto the gas being contained therein, the pore volume can be reduced bythe relatively high piston force utilized to force the casting materialinto the casting cavity during the final stage of the casting operation.

The apparatus according to the invention can be developed and modifiedin many different ways.

For example, a plurality of connections extending from a casting cavity10 to vacuum valves, and a plurality of filling level sensors, can beprovided in the die casting apparatus; in this case, the use of themethod according to the invention assumes that no changes occur in therelationships of the degrees of filling of the individual passages,which are disposed between the filling level sensors and the vacuumvalves. As is immediately apparent, visualization of the characteristiccurve of FIG. 2 is advantageous due to the good resolution thereof;however, it is also possible to display other characteristic curves. Itis further possible for the closing signal not to be generated basedupon the piston reaching the predetermined position (s₃) in FIG. 2, butrather by the piston moving at a predetermined speed, and the like.

Reference Number List

-   2 main body-   4 main body-   6 mold portion-   8 mold portion-   10 casting cavity-   12 cylinder-   14 piston-   16 shaft-   17 actuator-   18 filling conduit-   20 filling level sensor-   22 casting material-   24 vacuum opening-   26 vacuum valve-   28 vacuum pump-   30 solenoid-   32 motion sensor-   34 control device

1. A method for controlling a vacuum valve of a vacuum die castingapparatus, in which liquid casting material (22) is pressed by a piston(14) into an evacuated casting cavity (10), a predetermined fillinglevel of the casting material in the casting cavity is detected, and thevacuum valve (26) is disposed in a connection between a vacuum source(28) and the casting cavity (10) and the vacuum valve is actuated whenthe filling level is reached, such that the vacuum valve is closed whenthe casting material reaches a connection of the vacuum valve to thecasting cavity, characterised in that the movement of the piston (14) isdetected, the position at which the piston is disposed when the castingmaterial reaches the predetermined filling level is marked, and thevacuum valve (26) is closed when the piston has moved further in apredetermined manner from the marked position.
 2. A method as set forthin claim 1, characterised in that the speed at which the piston (14) isdisplaced is increased when the predetermined filling level is reached.3. A vacuum die casting apparatus including a casting cavity (10) thatis evacuatable via a vacuum valve (26) and, into which a liquid castingmaterial is pressable by means of a piston (14) actuated by an actuator(17), a filling level sensor (20) for detecting a filling level of thecasting material in the casting cavity, and a control device (34)connected to the filling level sensor for controlling the vacuum valve,characterised by a position sensor (32) connected to the control device(34) for detecting the movement of the piston (14), wherein the controldevice generates a closing signal for the vacuum valve (26) when thepiston, after reaching the position (s₁) at which the filling levelsensor (20) indicates a predetermined filling level of the castingcavity (10) with casting material, is displaced in a predeterminedmanner.
 4. A vacuum die casting apparatus as set forth in claim 3,characterised in that the vacuum valve (26) switches with apredetermined time delay from its open position into its closed positionin response to a closing signal.
 5. A vacuum die casting apparatus asset forth in claim 3, characterised in that the control device generatesthe closing signal for the vacuum valve (26) when the piston (14) hasbeen displaced a predetermined distance from the position (s₁), whichposition corresponds to the predetermined filling level.
 6. A vacuum diecasting apparatus as set forth in claim 3 or 4, characterised in that inthe control device (34), the position of the piston (14), which isdetected by the motion sensor (32), is differentiated with respect totime, and the time point at which the closing signal for the vacuumvalve (26) is generated is determined based upon the speed of the pistonand the instantaneous position of the piston.
 7. A vacuum die castingapparatus as set forth in one of claims 3 to 6, characterised in that acharacteristic curve is stored in the control device, which curveindicates the piston speed in dependence on the piston position.
 8. Avacuum die casting apparatus as set forth in one of claims 3 to 7,characterised in that a characteristic curve is stored in the controldevice (34), which curve indicates changes of the piston position overtime.
 9. A vacuum die casting apparatus as set forth in claim 7 or 8,characterised in that a reference characteristic curve is stored in thecontrol device and deviations between an actual characteristic curve andthe reference characteristic curve are displayed.
 10. A vacuum diecasting apparatus as set forth in one of claims 7 to 9 characterised inthat the characteristic curve used for determining the timing of theclosing signal is continuously updated.